Photo detector methods to reduce the disabling effects of displacement current in opto-couplers

ABSTRACT

This invention discloses the several means by which transient noise due to capacitance related displacement current can be excluded from the optical signal coming from a silicon detector used in opto-couplers. The exclusion of such noise permits a high degree of detector sensitivity which permits the use of low efficiency silicon based LEDs for opto-coupler applications.

This application claims the benefit of Provisional application Ser. No.60/316,862, filed Sep. 4, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of semiconductor photo detectorsused in opto-couplers, and particularly to reducing the disabling effectof displacement current in photo diodes used in opto couplers.

2. Description of the Related Art

Opto couplers are used to allow two electrical systems to communicatewith each other while remaining electrically isolated. Thiscommunication is achieved by sending light signals using an electricallyactivated light emitter, typically a Light Emitting Diode (LED), to aphoto detector which converts the light signals back into electricalsignals. The light passes through a transparent insulator therebyelectrically isolating the light emitter and its associated circuitryfrom the light detector and its associated circuitry. The associatedcircuitry of the LED side of the optocoupler can include an LED driver,amplifier, etc. The associated detector circuitry can include anamplifier, output drivers, A to D and D to A converters, etc.

Unfortunately, a displacement current can flow from the LED side of theopto coupler into the photo detector and cause an electrical output fromthe photo detector in addition to the electrical output produced by thelight. The displacement current is a consequence of the unavoidablecapacitance coupling between the LED side of the opto-coupler and thephoto detector. This displacement current flows when the voltage appliedto the coupler's LED and its surrounding electrically conductivestructures changes with respect to the voltage applied to the coupler'sphoto detector. The magnitude of the unwanted displacement currentflowing from the photo detector is dependent on the dV/dt or the ratechange in the voltage between the LED side and the photo detector. Thespurious detector signals produced by displacement currents cantherefore be disruptive to the normal communication process of theopto-coupler. Thus, there is a strong need to keep displacement currentout of the photo detector, especially if the light signal is weak as isthe case with silicon based LEDs.

Photo detectors associated with silicon integrated circuits aretypically realized using semiconductor junctions. The junction basedphoto detectors include PN diodes, bipolar transistors, SCRs, andTriacs.

In general terms, the displacement current is given by$I_{d\quad i\quad{sp}\quad l\quad a\quad c\quad e\quad m\quad e\quad n\quad t} = {C_{c\quad o\quad u\quad p\quad l\quad i\quad n\quad g}\frac{\mathbb{d}V}{\mathbb{d}t}}$where I_(displacement) is the displacement current or current flowinginto the capacitance, C_(coupling) is the capacitance between twoelectrically isolated conductors, and dV/dt is the rate change involtage between the two isolated conductors.

The relative magnitude of this undesirable current can be estimated.Assume that the transparent insulator of an opto-coupler has a thicknessof 300 μm and a relative dielectric constant of 2.8. The parallel platecapacitance per unit area is$C_{i\quad n\quad s} = \frac{ɛ_{i\quad n\quad s}}{t_{i\quad n\quad s}}$where C_(ins)=capacitance per unit area, ε_(ins)=permativity ofinsulator, and t_(ins)=thickness of insulator. For t_(ins)=0.03 cm (300μm) and ε_(ins)=2.8×8.854c−14 then C_(ins)=8.26 pF/cm². Since areasonable radius for a photo detector is 150 μm assume that thedetector area is equal to π×0.015² or 0.000707 cm². ThenC_(coupling)=5.84 fF. To make the calculation worst case, assume thatthe fringe field coupling is 50% of the parallel plate capacitance for atotal capacitance of 1.5×5.84 or 8.75 fF.

Assuming a transient voltage between the chip with the LED and the lightdetector of 10⁶ V/sec, then i_(coupling)=8.75 nA. Some opto-couplerspecifications show a “common mode” slew rate as high as 10⁹ V/sec,which produces a displacement current of 8.75 μA for a couplingcapacitance of only 8.75F.

For an opto-coupler using a silicon junction avalanche LED, a reasonablequantum efficiency is 10⁻⁵. Assuming a detector quantum efficiency of0.8, then for an LED current of 10 mA the photo current would be 80 nA.Although an 80 nA data signal current by itself can be readily detectedby an amplifier circuit, a superimposed spurious displacement current100 times greater in magnitude can make data extraction difficult anderror prone.

FIG. 1 shows an example of the cross section of a silicon basedopto-coupler. Two integrated circuits, 106 and 107, are shown separatedby a transparent insulator 108. An LED 111 is built into integratedcircuit 106 and emits light through the transparent insulator 108 to alight detector 112 of the receiving integrated circuit 107. Bond wire104 connects package lead 102 to integrated circuit 106 and bond wire105 connects package lead 103 to integrated circuit 107. Also shown isthe lead frame die attach plate 110 for integrated circuit 106 and thelead frame die attach plate 109 for integrated circuit 107. Package lead102 connects to integrated circuit 106 and establishes the basepotential of integrated circuit 106 and package lead 103 connects tointegrated circuit 107 and establishes the base potential of integratedcircuit 107. The base potential of each integrated circuit isestablished through a power supply connection to the substrate of eachintegrated circuit. The surface of each integrated circuit may containareas were the voltage is different from the base or substrate potentialby several volts.

Under normal operation, light 112 is emitted from LED 111 to lightdetector 112. However, as shown in FIG. 1, a large electrical spike canexist between the base potentials of integrated circuits 106 and 107.The rate change in the voltage difference produces a displacementcurrent 113 in the insulator 108 between integrated circuits 106 and107. Some of the displacement current 113 flows out of the lightdetector 112 potentially disrupting operation.

As can be appreciated by one normally skilled in the art, LED 111 couldalso be a discrete GaAsP LED.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a means by whichthe displacement current flowing between two electrically isolated butoptically linked semiconductor devices will not cause disruption in theoptical communication process between the two semiconductor devices.These methods include using two detectors for differential sensing,using a conducting transparent ground shield over the light detector,using a diffusion or an implanted top layer as a ground shield, andusing a MOSFET configured as a light detector with the gate serving as aground shield. Also, it is shown that more than one level of metal of anintegrated circuit can be used to shield interconnect circuitry fromdisplacement current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of an opto coupler illustratingdisplacement current flow between the two electrically isolatedsemiconductor devices.

FIG. 2 shows a cross section of a differential junction diode photodetector used to null out displacement current.

FIG. 3 shows a cross section of a junction diode photo detector with agrounded transparent conductor blocking displacement current.

FIG. 4 shows the cross section of a photo junction diode which uses anupper grounded implant to absorb displacement current.

FIG. 5 shows a cross section diagram of a photo MOSFET which uses thegate electrode as a displacement current shield.

FIG. 6 shows a partial cross section of a photo diode electricallyshielded by a transparent conductor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 shows an example of a differential light detector which can beused to null out a displacement current induced by the light sourceelements. Placed in a P type silicon substrate 200 is an N+ implant ordiffusion 201, a second N+ implant 203, and three P+ implants, 202A,202B, and 202C. The first N+ implant 201 forms an N+/P junction diodeand the second N+ implant 203 forms a second N+/P junction diode. P+implants 202A, 202B and 202C form substrate ties or a means ofconnecting metal conductors to the P substrate 200. Thus, the Psubstrate 200 areas near the N+ junctions 201 and 203 are robustly tiedto ground 205 via the P+ ties 202A, 202B, and 202C.

The diode junction formed with the N+ implant 201 is used as a photodetector in this example. Light 206A coming from the LED source passesthrough a transparent dielectric 209 such as SiO₂ and into the diodeformed by the N+ implant 201 thereby creating a photo current. Thesecond diode formed with the N+ implant 203 is identical in size andshape to the first diode formed with the N+ implant 201 but is shieldedfrom any light 206B that might come in over the diode from the LEDsource. The second diode formed with 203 is used as a voltage referencefor the photo diode formed with 201.

The light shield 204 covering N+ implant 203 can be any masked, opaqueinsulating layer such as a dyed photo resist. The material for the lightshield 204 should be thin as possible so as to minimize any differencebetween the displacement current 207 of the photo diode and thedisplacement current 208 of the reference diode 203. Also, the lightshield 204 can be made of a floating, opaque conductive material such asaluminum, copper, or a barrier metal such as tungsten. A floating,opaque conductor 204 can transmit the displacement current 208 whileblocking the light 206B. A conducting light shield 204 works the same asan insulating light shield 204 if, for the insulating light shield, theelectric field is uniform over the surface of the reference diodecomprising 203 and the electric field is normal or perpendicular to theinsulating light shield 204. For this case the electric potential isuniform over the surface of the insulating material and, hence, arelatively thin conducting light shield will have the same fielddistribution since the potential will be uniform in the shield.

The diodes formed by N+ implants 201 and 203 have a differential signaloutput 210. Light hitting the structure of FIG. 2 will cause the outputof a photo current well above the background junction leakage current onthe left output 210 lead that is connected to implant 201 whereas onlyjunction background leakage current will appear on the right output 210lead that is connected to implant 203. Thus, a differential signal on210 will result in response to illumination of the structure of FIG. 2.

However, if a rate change in voltage occurs between the structure ofFIG. 2 and the LED source a displacement current 207 will flow into 201and a displacement current 208 will flow into 203. Because of theaforementioned identical geometries of 201 and 203 displacement current207 and the displacement current 208 will be essentially identical.Thus, a displacement current will produce a change in the common modecurrent of the output 210 but no change in the differential signal.Thus, light 206A including stray light 206B will produce a differentialsignal at the output 210 but displacement current 207 and 208 will not.

It is also noted that substrate noise is also presented as a common modesignal on the output 210 and not as a differential signal. Thus, thisdetector construction is also useful for rejection of substrate 200noise.

As can be appreciated by one normally skilled in the art, the impuritypolarities of FIG. 2 can be reversed. That is, the P substrate can be Ntype, the substrate tie implants 202A, 202B, and 202C can be N type, theimplants 201 and 203 P type. Also, the diode structure of FIG. 2 can beimbedded into a well which is implanted into a substrate of oppositepolarity such as a the N well associated with a PFET of CMOS standardprocess using a P type substrate.

Another method of eliminating the displacement current between an LEDsource and the light detector is to use a transparent electrostaticshield. An Indium tin oxide electrostatic shield can be deposited over alight detector thus eliminating displacement current from appearing atthe output of the detector. Another material that can be used but withless transparency is polysilicon.

FIG. 3 shows a cross section of a shielded photo diode detector. Theshielded detector consists of a semiconductor substrate 300, an N+implant 301, transparent interlevel dielectric 309 such as SiO₂, a P+implant 302 to connect the substrate 300 to a metal lead 312, contactmetal 308 for the N+ implant, contact metal 311 for the P+ implant 302,a transparent conductor 303, a ground interconnect lead 312, and anelectrical ground point 305.

Light 306 from the LED propagates through the transparent conductor 303,through the interlevel dielectric 309 such as SiO₂, through the thin N+implant layer 301, and into the substrate region below the N+ implant301 where the light is absorbed. The absorption of light in thedepletion region, whose boundaries are the N+ implant 301—P substrate300 interface and the boundary 304, produces an electrical currentbetween the N+ implant 301 and the P substrate 300. Also, photogenerated carriers below but near the depletion region boundary 304 cancontribute to the electrical current by diffusion. The photo generatedelectrical current flows out through lead 310 and is referenced toground 305.

A displacement current 307 flows through the transparent conductor 303to ground 305 rather than through the N+ implant layer 301 to the photocurrent output 310.

It is noted that the transparent conductor 303 adds some undesirablecapacitance to ground for the output node 301 via electrostatic couplingbetween the N+ implant layer 301 and the transparent conductor 303. Thiscapacitance can be reduced by increasing the thickness of the interleveldielectric 309.

To determine the effect of the transparent conductor may have on thedetector capacitance assume that the substrate doping is 1e15/cm³ thenthe capacitance per unit area is 1e4 pf/cm² or 0.1 fF/μm² at zero bias.For an oxide thickness of 1 μm, the capacitance per unit area is 3453pF/cm² or 0.0345 fF/μm². Thus, an ITO layer placed above a diode doped1e15/cm with an intervening SiO₂ dielectric thickness of 1 μm will add34.5 percent more capacitance to the diode at 0 bias. A transparentconductor made of ITO can be expected to absorb approximately 10% of thelight passing through it.

As noted earlier, a thin polysilicon layer can also be used in place ofthe ITO. The MOSFET gate polysilicon thickness for sub-micron processescan be as thin as 0.25 μm. To determine how much light is absorbed in apolysilicon shield it will be assumed that the absorption coefficientfor polysilicon is the same as that for silicon. The light emission peakwavelength of a silicon avalanche LED is about 590 Å which is yellow.The silicon absorption coefficient for this wavelength is 2 μm. Thus,yellow light is reduced to 88% from its initial value after passingthrough 0.25 μm of silicon. Even thinner thicknesses for polysilicon canbe used but may not be useful as the gate material of MOSFETs. Thus,polysilicon can be a useful electrostatic shield for a photo diode dueto it high transmission of light if thin enough.

An implanted N Well photo diode with a displacement current groundshield implant can also be made. In this construction, a three terminalN Well based diode is used as shown in FIG. 4. The N Well connects to anoutput lead 403 using an N+ tie implant 401 and the substrate connectsto ground 407 using a P+ implant 402. The top P+ implant layer 404 andthe P substrate 400 are connected to ground 407 while the N Well 405 isused as the signal node which is output on 403. The P+ implant layer 404is made thin to minimize the absorption of light 406. Thus, displacementcurrent 407 will flow through the top P+ implant layer 404 to ground 407and will not disturb the photo current flowing between the N Well 405and the P+ implant layer 404 and between the N Well 405 and the P−substrate 400. The P+ implant 404 and N Well 405 can be the same as thatused to make the drain/source implant and N Well implant of a PFET,respectively, thereby allowing the structure of FIG. 4 to be madewithout any additional fabrication steps over that of a standard CMOSprocess.

FIG. 5 shows the construction of a PFET 514 based photo detector. Inthis construction, a PFET 514 is used to create a shielded photo diode.The gate polysilicon 503 is used as the transparent electrostatic shieldand is connected to ground 505. The drain implant 504A and the sourceimplant 504B of PFET 514 are connected together and are biased via node515. P+ implants 502 in the P substrate 500 are used to tie thesubstrate 500 to ground 505. An N+ implant 501 is used to tie the N Well516 to the detector output 508.

An example of a biasing circuit for the drain 504A and source 504B of514 is also shown in FIG. 5 and is comprised of a second PFET 512 and aload resistor 511 connected to the drain of PFET 512. The source of PFET512 is connected to ground 505, the gate of PFET 512 is connected to thedrain of PFET 512 which is connected to the load resistor 511. Thesecond end of the load resistor is connected to a positive supplyvoltage, Vdd 513.

Note that the PFET 514 is biased into the inversion regime so that thereis not only a photo carrier collection zone associated with the N Well516-Substrate 500 junction depletion region that extends from 509A to509B but also with the depletion region 510 associated with the PFETinversion layer 517. The P+ Drain 504A/Source 504B implants have to bebiased such that the PFET 514 is in inversion and such that the N Well516-substrate 500 junction and the inversion layer 517-N Well 516junction are reversed biased. One way to generate Drain 504A/Source 504Bbias voltage on node 515 is to use the second PFET 512 as referencevoltage source as shown. With a large value of R_(load) 511, the output515 of the reference PFET 512 will be slightly above its thresholdvoltage with respect to ground 505 and includes the body effect onthreshold voltage. Assuming that the threshold voltage of 512 matchesthat of 514 then the voltage on 515 will cause an inversion layer 517 toform in PFET 514. Node 508 is the photo diode output and can be biasedanywhere from the reference voltage on node 515 to Vdd 513. Note that asbias voltage on photo current output 508 is increased above thereference voltage on 515, the depletion boundaries 509A, 509B, and 510will move causing expansion of the depletion regions wherein photogenerated carriers are generated and electrically collected. Thus,collection efficiency and response time suggest the bias voltage on theoutput node 508 be close to Vdd.

An alternate operating scheme for the PFET based light detector of FIG.5 is to use the Drain 504A/Source 504B implants as the signal node withthe N well 516 and gate 503 tied to a quite or filtered ground. Thisconfiguration would prevent substrate 500 noise from interfering withsensing. In this mode photo carriers generated in the depletion regionassociated with the inversion 517 would be collected by the inversionlayer 517. The photo current would therefore flow from the N well 516 tothe inversion layer 517. The draw back of this operating configurationis the inversion layer 517 to gate 503 capacitance.

As can be appreciated by one normally skilled in the art, the impuritypolarities and voltage polarities shown in FIG. 5 can be reversed.

FIG. 6 shows a partial cross section of a photo diode electricallyshielded by a transparent conductor 603 which can be made of indium tinoxide or polysilicon. The N+ implant 601 forms the cathode and collectsthe photo current to be output from the photo diode while the Psubstrate 600 forms the anode of the photo diode and is connected vialead 602 to ground 605. The transparent conductor 603 is also tied toground 605 so that any displacement current 604 from the LED source isshunted to ground 605. Transparent interlevel oxide 606 such as, but notlimited to SiO₂, is used to insulate interconnect metal, polysiliconused for MOSFET gates, and the transparent conductor 603 which may alsobe polysilicon. Terminal 607 is a metal contact to the outputinterconnect metal of the photo diode. For the process used to make thephoto diode of FIG. 6 it is assumed that at least a second level ofmetal is available. The second level of metal is used to make anelectrostatic shield 608 for the terminal 607 and any areas of the N+implant 601 not covered by the transparent conductor 603 therebyshunting displacement current away from the photo output signal terminal607. This shielding method can also be applied to any interconnect metalconnected to the photo detector's output. Thus, the use of a second orhigher level of metal tied to ground in combination with a transparentconductor 603 can prevent to a very high degree any displacement currentfrom flowing into the photo signal node including any interconnect metalgoing to a sense amplifier. It is noted that interconnect shieldingmethod of FIG. 6 can be applied to any of output nodes of the photodetectors described herein.

1. A light detector having a differential output, comprising: a firstjunction diode for converting incoming light into photocurrent; a secondjunction diode; a light shield substantially covering the secondjunction diode; a first output lead coupled to the first junction diode;and a second output lead coupled to the second junction diode; whereinthe first output lead and the second output lead form the differentialoutput of the light detector.
 2. The light detector of claim 1, whereinthe first junction diode comprises a first N+ implant in a P typesilicon substrate and the second junction diode comprises a second N+implant in the P type silicon substrate.
 3. The light detector of claim1, wherein the second junction diode is substantially identical to thefirst junction diode.
 4. The light detector of claim 1, wherein thelight shield comprises an opaque insulating layer.
 5. The light detectorof claim 4, wherein the opaque insulating layer is made of dyed photoresist.
 6. The light detector of claim 1, wherein the light shieldcomprises an opaque conductive material.
 7. The light detector of claim6, wherein the opaque conductive material is aluminum.
 8. The lightdetector of claim 6, wherein the opaque conductive material is copper.9. The light detector of claim 6, wherein the opaque conductive materialis tungsten.
 10. A shielded light detector, comprising: a photo diode; adielectric layer formed over the photo diode; a transparentelectrostatic shield formed over the dielectric layer and the photodiode; and a ground lead coupling the transparent electrostatic shieldto an electrical ground.
 11. The light detector of claim 10, wherein thetransparent electrostatic shield comprises indium tin oxide.
 12. Thelight detector of claim 10, wherein the transparent electrostatic shieldcomprises polysilicon.
 13. The light detector of claim 10, wherein thephoto diode comprises an N+ implant in a P type substrate.
 14. A methodfor using a PFET as a light detector, the PFET comprising a polysilicongate and a source and drain in an N well, wherein the N well is in a Ptype substrate, the method comprising: coupling the gate of the PFET andthe P substrate to a ground; coupling the N well to an output of thelight detector; biasing the output between a reference voltage and apower supply voltage; and biasing the source and drain of the PFET toform an inversion layer in the PFET.